Semiconductor diode lasers such as phased array diode lasers are a preferred source of radiation for pumping fiber lasers because of their high power efficiency and availability at useful wavelengths. However, the output of such diode lasers is multimode and non-circular in cross section. Multimode beams are generally undesirable and non-circular cross-section make it difficult to match the pump output to the fiber laser input. This problem has been addressed by using a double clad fiber laser in which the single-mode core waveguide is surrounded by a second waveguide which accepts the multimode radiation and uses that multimode radiation to pump the single-mode core to produce the fiber laser action in that single-mode core. This technique is taught in U.S. Pat. No. 4,815,079 issued Mar. 21, 1989.
This solved two problems: first, the greater available power of the laser diodes in undesirable multimode form could be used to efficiently pump a single-mode fiber to obtain the desired single-mode laser output. Further, the increased area of the second multimode waveguide relative to the area of the first single-mode waveguide also extended by approximately the same ratio the length of the single-mode fiber over which absorption occurs. This distributed absorption similarly distributes the heat over a longer length, thereby increasing the acceptable pump and output power levels. Without such broader distribution of the heat the temperature could rise to elvels which would impede the laser action.
However, there are still power limiting constraints on the double clad fiber laser. First, there is a limit on how much laser power the single-mode waveguide can sustain before optical damage occurs to the core. To prevent this the power density in the core must be decreased. Both this can only be accomplished by increasing the cross-section area of the core and large cores will support undesirable high order modes. Second, the present developments in semiconductor lasers, such as phrased array diode lasers, are tending toward ever larger apertures inconjunction with increasing laser power. This requires that the second, multimode waveguide be made larger to match the aperture of those lasers. But when the ratio of the area of multimode waveguide to the area of the single-mode waveguide increases, the absorption per unit length decreases proportionally. Therefore the length of the fiber laser has to be increased to absorb the additional power. The longer fiber length constitutes a loner laser cavity which introduces more loss and can reduce the efficiency of the laser action or defeat the laser action entirely.
thus the problem devolves to making the single-mode fiber optical laser long enough to safely absorb all the required power and short enough to avoid laser cavity losses that would suppress or defeat laser action. Presently, increasing power absorption in the single-mode fiber while minimizing lengthening of the fiber can be accomplished by increasing the diameter of the fiber, but this permits greater multimode propagation, which is undesirable.